1. Field of the Invention
The present invention relates to a mixed ionic conductor used for an electrochemical device such as a fuel cell or a sensor.
2. Prior Art
As an ionic conductor material, there are three types, a solution type, a molten salt type or a solid state type. Almost all of devices based on an electrochemical reaction such as a cell, a sensor or a fuel cell use mainly either one of the solution type conductor and the molten salt type conductor having a conductivity higher than that of the solid state type conductor. However, it has been desired to realize an electrochemical device provided with a solid state conductor which is not evaporated, lost or leaked out.
A solid material of an ionic crystal shows a variety of ion conductivity due to a diffusion of ions in the crystal at a temperature close to the melting point. In 1914, Tubant and others found Silver ion conductive material (AgI) which has a ion conductivity comparable to that of the solution at a temperature much lower than the melting point. In the age of 1960, there was discovered a stabilized zirconia which is an oxygen ion conductor and Na-.beta.-alumina which is a sodium ion conductor. Various researches have been directed to a discovery of an ion conductor showing an ion conductivity which is large at a temperature as low as possible. Since then, much attention has been paid to the ion conductor applicable for various kinds of sensors, solid state cell, various kinds of memory devices, display devices or fuel cell.
On the other hand, there is reported a bismuth oxide and a cerium oxide which have a high ion conductivity above 600.degree. C. except for the stabilized zirconia. However, since these materials are poor in the resistance to a reducing atmosphere and are unstable in the chemical reaction, these materials are not suitable for the electrochemical device especially for a fuel cell. There are various papers to report ionic conductors which comprise a variety of complex oxides but most of such oxides show a low ion conductivity and a poor chemical stability. However, it is found that some oxides in a perovskite structure show a high chemical stability and a high ion conductivity due to mixed ion conductivity of the protons and oxide cations, which conductivity is close to that of YSZ (yttrium stabilized zirconia) at 1000.degree. C. A variety of researches have been directed to BaCeO.sub.3-.alpha. added with a third additives among the above oxides in a perovskite structure. In connection with the third additives, the following ion conductivity is reported in the literature of H. Iwahara et al, J. Electrochem. S, 135,(1988)529: At 1000.degree. C., 8.times.10.sup.-2 S/cm (BaCe.sub.0.9 (Y, Nd and Sm).sub.0.1 O.sub.3-.alpha.); at 800.degree. C., 4.3.times.10.sup.-2 S/cm and at 600.degree. C., 1.6.times.10.sup.-2 S/cm (BaCe.sub.0.85 Gd.sub.0.15 O.sub.3-a).
N. Bonanos and others carried out a systematic research on BeCe.sub.1-x Gd.sub.x O.sub.3-.alpha. complex compound and filed a UK Patent Application No. 8716147 which was published under Publication No. GB 2206571 A. However, sintered samples used in the reseach do not reach to 92% of theoretical density. The synthesizing process for the sintered material has a fair problem. The disclosure of Solid State Ionics, 35,(1989)179. shows that the highest ion conductivity of the sintered material is obtained with x=0.15 in the above complex formula.
However, there has been no report on the ionic conductor which is chemically stable to a same degree as that of YSZ and has a ion conductivity higher than that of YSZ. In order to make an electrochemical device in a solid state where no evaporation, loss or a leak of the electrolyte occur, it is necessary to find a material having a high chemical and thermal stability as well as a high ion conductivity and its synthesizing method. Therefore, an object of the present invention is to provide an ionic conductor provided with the above properties.
Another object of the present invention is to provide the method for synthesizing the ionic conductor material.
Other objects of the present invention will appear in the course of the description thereof follows.